The present invention generally relates to welding processes and materials. More particularly, this invention relates to a process for welding precipitation-strengthened superalloys that are prone to cracking when welded.
Superalloys are used in the manufacture of components that must operate at high temperatures, such as buckets, nozzles, combustors, and transition pieces of industrial gas turbines. During the operation of such components under strenuous high temperature conditions, various types of damage or deterioration can occur, including wear and cracks. Because the cost of components formed from superalloys is relatively high, it is more desirable to repair these components than to replace them. For the same reason, new-make components that require repair due to manufacturing flaws are also preferably repaired instead of being scrapped.
Methods for repairing nickel-base superalloys have included gas tungsten arc welding (GTAW) techniques. GTAW is known as a high heat input process that can produce a heat-affected zone (HAZ) in the base metal surrounding the weldment. A filler is typically used in GTAW repairs, with the choice of filler material being between a ductile filler or a filler whose chemistry closely matches that of the base metal. An advantage of using a ductile filler is a reduced tendency for cracking in the weldment. On the other hand, a significant advantage of using a filler whose chemistry closely matches the base metal is the ability to more nearly maintain within the component the desired properties of the superalloy base material.
Directionally-solidified (DS) and single-crystal (SX) castings formed of precipitation-strengthened nickel-base superalloys have proven to be particularly difficult to weld. Though an equiaxed (EA) precipitation-strengthened nickel-based superalloy filler wire having a composition similar to that of the superalloy base material being welded would provide an optimum weld repair, the result is often solidification shrinkage, hot tears, and cracking during and after the welding processes, and strain age cracking due to gamma prime (γ′) precipitation (principally Ni3(Al,Ti)) during post-weld vacuum heat treatment. Cracking is particularly likely in the termination region of the weldment. Further complicating the termination of the weldment is the typical geometry of the superalloy article being welded.
In view of the above, improved methods are required for welding precipitation-strengthened superalloys that will yield crack-free weldments.